Semiconductor memory devices may be classified into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. Volatile semiconductor memory devices can perform read and write operations at high speed, while contents stored therein may be lost at power-off. Nonvolatile semiconductor memory devices may retain contents stored therein even at power-off. The nonvolatile semiconductor memory devices may be used to store contents which must be retained regardless of whether they are powered. The nonvolatile semiconductor memory devices may include a Mask Read-Only Memory (MROM), a Programmable ROM (PROM), an Erasable Programmable ROM (EPROM), an Electrically Erasable Programmable ROM (EEPROM), and the like.
A representative nonvolatile memory device may be a flash memory device. The flash memory device may be widely used as a voice and image data storing medium within information appliances such as a computer, a cellular phone, a PDA, a digital camera, a camcorder, a voice recorder, an MP3 player, a handheld PC, a game machine, a facsimile, a scanner, a printer, and the like.
A multi-bit memory device storing multi-bit data in one memory cell has become increasingly common according to an increasing need for higher integration levels. It is desirable to manage a threshold voltage distribution of multi-bit memory cells in order to improve the reliability of these cells.